Researchers have identified microscopic kidney lesions in horses with oleander poisoning, but they say they are unable to serve as a diagnostic marker to differentiate it from other disease processes or causes of death.
Oleander (Nerium oleander) is a hearty evergreen plant from the Mediterranean region. It has been long associated with intentional and unintentional poisoning of humans and animals.
Oleander has become a popular ornamental plant throughout the western and southern United States and elsewhere in the world.
All parts of the plant are poisonous, containing varying amounts of cardiac glycosides known as cardenolides, the most common of which is oleandrin. Oleander is bitter tasting, and most animals avoid consuming it unless no other forage options are available.
The most common route of exposure in livestock and horses is by accidental incorporation of oleander into hay bales, or in cut forage being thrown into a pasture or pen.
As it dries, the plant becomes less bitter but remains toxic, and the animals are less likely to avoid the clippings.
The common clinical signs of oleander poisoning in horses include colic, a weak pulse, congested mucous membranes, slow capillary refilling time, a fast heart rate, high-grade heart blockage, depression, lethargy, tremors, acute kidney failure, and death. It is not unusual for owners to find animals dead without prior clinical signs being seen.
Exposure to oleander is confirmed by finding leaves in the gut and by testing serum, plasma, urine, liver, or gastrointestinal contents for oleandrin.
A definitive postmortem diagnosis of oleander intoxication should consider the history of exposure, clinical signs, and microscopic evidence of cardiomyocyte cell degeneration and death in the heart. However, in some cases, there may be minimal microscopic heart changes.
In goats, sheep, and cattle, kidney lesions have been well described, and appear to occur commonly and with moderate to marked severity in oleander intoxication. Kidney lesions in horses related to oleander ingestion have been only briefly mentioned in previous research.
Chelsea Sykes and her fellow researchers with the California Animal Health and Food Safety Laboratory System set out to describe the type, extent, and frequency of kidney lesions in horses that died with a diagnosis of oleander poisoning. They also sought to determine whether microscopic kidney-related lesions can be used as a postmortem diagnostic marker of oleander poisoning in the species.
The researchers, writing in the journal Animals, reviewed 21 cases of spontaneous oleander poisoning in horses. They evaluated kidney samples microscopically, comparing microscopic lesions with those detected in 10 horses who died or were euthanized for other reasons.
The study team found microscopic kidney lesions, principally mild to moderate tubular changes, as well as mineralization and congestion, in horses with oleander poisoning. “Most of these changes match the descriptions of lesions previously noted in other species,” they said, “although with less frequency and severity.”
However, similar lesions were found in horses that died spontaneously of different causes, or were euthanized.
“We concluded that microscopic renal lesions may be detected in horses with oleander poisoning but they cannot be used as a diagnostic marker that allows differentiation from other disease processes or causes of death,” the researchers reported.
The study team comprised Sykes, Francisco Uzal, Aslı Mete, Jennine Ochoa, Michael Filigenzi, Robert Poppenga and Javier Asin.
Sykes, C.A.; Uzal, F.A.; Mete, A.; Ochoa, J.; Filigenzi, M.; Poppenga, R.H.; Asin, J. Renal Lesions in Horses with Oleander (Nerium oleander) Poisoning. Animals 2022, 12, 1443. https://doi.org/10.3390/ani12111443